This course gives an introduction on theoretical and practical aspects of laser beam characterization based on the concept of second order moments. This concept is the fundament of the current ISO 11146 standard and is important for both laser development and laser application. The first, more theoretical part of the course presents the definition of the second order moments as a measure for a beams transverse extent. Advantages and disadvantages against other beam width definitions are discussed considering some typical beam profiles. The propagation law of the second order moments through aberration-free first order optical systems is presented as well as the relationship to other beam propagation parameters like divergence, Rayleigh length, wavefront, astigmatism, etc. A geometric and intrinsic beam classification derived from the propagation properties through stigmatic and simple-astigmatic optical systems will be introduced. The second part of the course deals with all aspects of experimental beam characterization. Different measurement schemes are presented and discussed in detail. Emphasis is given to CCD camera based measurements, including discussion of typical image distortions and their compensation, determination of suitable regions of interest, and determination of the offset level. The final part of the course gives a short outlook on beam characterization beyond the current standards, demonstrating how a complete characterization of partially coherent beams can be achieved by interferometric and non-interferometric measurement setups.